Ambient dose equivalent H*(10)
The body related dose quantities (radiation-weighted and effective dose) are not directly measurable and, therefore, cannot be used directly in radiation protection monitoring. For that reason operational quantities have always been applied for the assessment of effective dose or mean doses in tissues or organs. Operational quantities are aimed at providing a conservative estimate or upper limit for the value of the protection quantities related to an exposure, or potential exposure of persons under most irradiation conditions. They are often used in practical regulations or guidance instead of the protection quantities. These quantities complement the system of quantities generally applied in radiological protection. As shown in the figure, different types of quantities are used for internal and external exposure situations.
For radiation monitoring in cases of external exposure (area or individual monitoring) operational dose equivalent quantities are defined. Operational quantities are used for monitoring external exposures because
- protection quantities are not directly measurable,
- for area monitoring point quantities are needed, effective dose is not appropriate in area monitoring, because in a non-isotropic radiation field its value depends on the orientation of the human body in that field, and
- instruments for radiation monitoring need to be calibrated in terms of a measurable quantity for which calibration standards exist.
Due to the different tasks in radiological protection, including area monitoring for controlling the radiation in work places and for defining controlled or restricted areas and individual monitoring for the control and limitation of individual exposures, different operational dose quantities have been defined. While measurements with an area monitor are mostly performed free in air, personal dosemeters are usually worn at the body. As a consequence, in a given situation, the radiation field "seen" by an area monitor free in air differs from that "seen" by an personal dosimeter worn on a body where the radiation field is strongly influenced by the backscatter and absorption of radiation in the body. The use of different operational dose quantities allows for such phenomena. For the different tasks of monitoring of external exposure the following quantities are defined:
With respect to the application of the operational quantities the ICRU (1993) has stated that H*(10) and Hp(10) are designed for monitoring strongly penetrating radiation, e. g. photons (above about 12 keV) and neutrons, while Hp(0.07) is applied for monitoring weakly penetrating radiation, e. g. α- and β-particles. Furthermore, Hp(0.07) is also used for monitoring the doses to the extremities from all ionising radiation.
In Nucleonica applications such as Dosimetry & Shielding and e-Ship, the ambient dose equivalent H*(10) is denoted by H10. The quantity h10 is the ambient dose or dose rate equivalent per unit activity.